1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. In particular, the present invention relates to a semiconductor device in which zinc oxide is used as a semiconductor material and to a method for manufacturing the semiconductor device.
2. Description of the Related Art
Zinc oxide (ZnO) is one type of II-VI compound semiconductor. The band gap energy of ZnO can be changed by making ZnO into a mixed crystal with MgO, CdO, or other suitable materials, and ZnO can have a multilayer structure of quantum well and other suitable structures. Furthermore, since the bond energy of an exciton is very large, ZnO is suitable for use in light-emitting devices. Since ZnO is transparent in the visible range, ZnO is also suitable for use in transparent thin film transistors for driving liquid crystal displays.
Meanwhile, ZnO has a wurtzite structure (hexagonal system). As shown in FIG. 9, ZnO has no center of symmetry in the c axis direction (vertical direction) and has polarity based on a molecular structure.
That is, ZnO has zinc-polarity (+c polarity) in which three bonds bonding to a Zn atom 51 point downward and three bonds bonding to an oxygen atom 52 point upward, as shown in FIG. 9(a), and oxygen-polarity (−c polarity) in which three bonds bonding to a Zn atom 51 point upward and three bonds bonding to an oxygen atom 52 point downward, as shown in FIG. 9(b).
Here, the above-described polarity refers to an orientation of the above-described bond and does not refer to an element terminating the surface.
It has been reported that a ZnO thin film having the oxygen-polarity was previously formed by a PMBE (plasma-assisted molecular-beam epitaxy) method on a sapphire substrate (APPLIED PHYSICS LETTERS, Vol. 80, No. 8, pp. 1358-1360 (2002); hereafter referred to as “first known technology”).
It has been reported that a film of GaN having Ga-polarity was formed on a sapphire substrate and, by controlling the film formation conditions, a ZnO thin film having the zinc-polarity or the oxygen-polarity was formed on the above-described GaN (APPLIED PHYSICS LETTERS, Vol. 77, No. 22, pp. 3571-3573 (2000); hereafter referred to as “second known technology”).
In addition, another technology has been reported, in which the polarity of a piezoelectric film of ZnO formed on a substrate was able to be specified (Japanese Unexamined Patent Application Publication No. 2001-144328; hereafter referred to as “third known technology”), as another known technology.
In the above-described third known technology, a piezoelectric film (ZnO film) having a + surface (zinc-polarity) or a − surface (oxygen-polarity) can be formed in accordance with the type of substrate, and the polarity of the piezoelectric film of ZnO formed on a substrate is controlled by changing the film formation conditions, e.g., a heating temperature of the substrate.
With respect to the above-described first known technology, it has been determined by Coaxial Impact Collision Ion Scattering Spectroscopy (CAICISS) that the ZnO thin film formed on the sapphire substrate has oxygen-polarity. However, substantially hexagonal crystal grains remain in such a ZnO film, the surface shape becomes uneven and, thereby, the desired surface smoothness of the ZnO thin film cannot be obtained.
That is, since the ZnO thin film formed by the first known technology has poor surface smoothness, where a semiconductor device is formed using the ZnO thin film, a current passes through grain boundaries, and a concentration of electric field occurs on convex portions of crystal grains. Consequently, the operation of the device may become unstable, or the device may be destroyed.
According to the second known technology, the polarity of the ZnO thin film can be controlled by changing the film formation conditions. In this manner, the ZnO thin film having the zinc-polarity or the oxygen-polarity can be formed on GaN. However, the substrate temperature increases during the film formation of ZnO on GaN and, thereby, Ga, which is an element of GaN, may diffuse into the ZnO thin film.
Since Ga functions as a donor to ZnO, if Ga diffuses into the ZnO thin film, the resistance of ZnO is reduced.
Furthermore, it is difficult to control the above-described diffusion and, therefore, variations may occur in the device characteristics of the semiconductor device.
In the above-described second known technology, since there is lattice mismatch between GaN and ZnO, lattice defects are introduced to mitigate the lattice mismatch. As a result, the crystallinity of the ZnO thin film is deteriorated and, thereby, deterioration of the electric characteristics occurs.
The above-described third known technology discloses the formation of the piezoelectric thin film having the zinc-polarity or the oxygen-polarity. However, there is no disclosure with respect to the influence exerted by the polarity on the surface shape and the electric characteristics of the thin film. Furthermore, since the material for the substrate is different from the material for the piezoelectric film, deterioration of the crystallinity may occur due to the lattice mismatch as in the second known technology, and there is a problem in that highly reliable, desirable, and excellent electric characteristics cannot be obtained.